a) Field of the Invention
The invention relates to a furnace with liquid-cooled grate elements which each have an inflow and a return for a coolant.
b) Description of the Related Art
Liquid-cooled, in particular water-cooled grate elements for furnace grates have been known for a long time from WO 96/29544 A1 and DE-C-624 892. The first publication discloses the arrangement of a container which is open to the atmosphere, but which allows only a connection of the return to the atmosphere. By contrast, the inflow is fed via a feed pump, with the result that the pressure of the coolant in this region, and also the throughflow quantity, are determined by the work of this feed pump and by the regulating valves located downstream of this pump. The second publication discloses a furnace grate, in which a container open to the atmosphere is provided at the upper end, but this container does not serve as a condensation device, instead making it possible for low-pressure steam to escape into the atmosphere. The amount of cooling of the cooling medium is more or less random in this combustion grate, since the volumetric flow of the primary air, which serves as a recooling medium for the cooling liquid, cannot be varied as desired. As is known, the primary air supplied must be governed by what is happening on the grate in terms of combustion and can therefore in no way bring about in the circulation system a defined condensation of steam which has possibly been formed. One disadvantage of relatively modern furnaces is that a comparatively high outlay in regulating terms has to be accepted, in order to ensure, on the one hand, sufficient cooling of the grate elements and, on the other hand, the necessary safety in the event of the action of excessive heat on the grate elements.
The primary object of the invention is to provide a furnace with a cooling system for the grate elements, which dispenses with a regulating device and a feed device for the circulation of the coolant and in which, above all, there is no need for any means for maintaining safety against excessive pressure.
Proceeding from a furnace of the type explained in the introduction, this object is achieved, according to the invention, in that the inflow and the return are connected to a condensation device open to the atmosphere, in that the inflow has arranged in it a U-shaped cooling-liquid seal, one leg of which has a liquid head which generates an arbitrarily selected maximum pressure in the system, and in that the other, shorter leg is connected to a central distributor for the individual grate elements.
An especially preferred refinement, which serves particularly for operating reliability, is characterized in that the upper end, connected to the central distributor, of the shorter leg lies below the lowest point of the coolant flow of the lowest grate element by a selected safety height amount.
The condensation device open to the atmosphere ensures that, even when the coolant is evaporated completely, there can be no higher pressure generated in the circulatory cooling system than that predetermined by the freely selectable liquid head of the longer leg of the liquid seal. In practice, at the present time, a liquid head of 4.85 m above the lowest point of the coolant flow in the lowest grate element will be selected, in order to prevent the excess pressure in the cooling system from exceeding 0.5 bar, since this system otherwise comes under the steam boiler decree which has different safety regulations. The distance between the lowest flow level of the lowest grate element and the upper end, connected to the central distributor, of the shorter leg is designated as the safety height amount and indicates that liquid head which generates in the U-shaped liquid seal a pressure intended to counteract a reverse flow in the cooling system, even when, due to the evaporation of the coolant, a high degree of bubble formation occurs in the grate element in the event of locally particularly strong heat radiation striking the latter. In practice, for safety reasons, this safety height amount is selected in such a way that it corresponds to twice the value of the height difference of an inclined furnace grate between the highest and the lowest point of the coolant flow in this furnace grate.
In order to provide uniform pressure differences between each grate element and the associated central distributor and therefore uniform flow conditions in the individual grate elements, according to an advantageous development of the invention there is provision for the central distributor to be arranged below the fluidically parallel-connected grate elements of the grate stages and in the longitudinal direction of the furnace grate with a vertical clearance which is uniform over the length of the entire furnace grate and is smaller than the safety height amount.
For the same reason, too, in a further refinement of the invention, the return has a central collector for the individual fluidically parallel-connected grate elements of the grate stages, which is arranged below the grate elements and in the longitudinal direction of the furnace grate with a vertical clearance which is uniform over the length of the entire furnace grate and is smaller than the safety height amount. The arrangement both of the central distributor and of the central collector with a vertical clearance relative to the furnace grate, which is smaller than the safety height amount, is provided because operational variations make it necessary, under some circumstances, to vary the safety height amount. Even in such a case, it should be ensured that the central collector and the central distributor have a smaller vertical clearance relative to the furnace grate than corresponds to the safety height amount. This central collector and this central distributor are permanently installed and their height can hardly be varied subsequently, which is not true to the same extent of the connection to the shorter leg of the U-shaped cooling-liquid seal, the said connection defining the safety height clearance.
In order to ensure that the flow velocity through all the grate elements is essentially the same and the result is the necessary pressure gradient for a direction of flow from the central distributor via the grate elements to the central collector, according to an advantageous development of the invention there is provision for a restrictor to be installed in each outflow line between the grate element and the central collector.
Since the grate elements receive relatively little cooling liquid, but a particular liquid reservoir is necessary so as always to have sufficient cooling liquid available in the event of excessive evaporation, in a further advantageous refinement of the invention there is provision for the second, short leg of the U-shaped cooling-liquid seal to have an additional storage volume for cooling liquid.
A preferred refinement for implementing a liquid reservoir is characterized, according to the invention, in that the short leg of the U-shaped cooling-liquid seal is designed as a container, into which the longer leg of smaller diameter penetrates and reaches near to the bottom of the short leg, in that the upper closed end reaches to just below the lowest point of the lowest coolant flow of the lowest grate element, and in that a branch to the central distributor emanates below the highest point of the container. Advantageously, at the same time, the cylindrical container is higher than corresponds to the geodetic height of the short leg, that is to say the cylindrical container extends beyond the branch to the central distributor.
So as to return, again, the entire cooling liquid present in the cooling system, in a development of the invention the central collector, starting from its lowest point, is connected via a line to a condensate collecting container. The cooling liquid can be introduced from here into the system again, in that the condensate collecting container is connected to the condensation device via a pump and a line. It is particularly expedient, in this case, that, according to the invention, the line opens with a spray nozzle into the condensation device.
If, in a further refinement of the invention, the condensation container is provided with a cooling device, the condensed cooling medium can then be returned in cooled form into the condensation device. This affords the possibility that, in a development of the invention, the condensation device is designed as a surface condenser with water-cooled cooling bodies and with a connectable wet-condensation means. The connectable wet-condensation means is formed, in this case, by the spray nozzle, by means of which cooled condensate is sprayed out of the condensate collecting container. This wet-condensation means, in which the steam returned to the condensation device condenses on the cooled water droplets, to some extent still ensures the circulation of cooling liquid even if the water-cooled tubes of the condensation device were to experience a fault.
If, in a further advantageous refinement of the invention, the condensation device is capable of being shut off relative to the atmosphere and of being connected to a vacuum source, the cooling system of the furnace can be put into operation in a particularly simple way as a result. In this case, due to the fall in pressure in the steam space of the condensation device, the same vacuum is generated in the central collector, with the result that the coolant flows out of the grate elements to the central collector according to the fall in pressure, this start of flow also being assisted in that so-called starting burners, which cause heat radiation to strike the furnace grate, are ignited in the furnace space above the furnace grate. The cooling medium located in the grate elements is thereby heated and, where appropriate, even evaporated, with the result that the cooling system is set in motion in the manner of gravity heating.
The invention is explained in more detail below with reference to an exemplary embodiment illustrated in the drawing.